In next-generation packet-based wireless systems like UMTS Long-term Evolution, VoIP will be an important service to be supported (UMTS=Universal Mobile Telecommunications System). However, VoIP data has unique traffic characteristics that differ from other packet data services. VoIP packets are very small, equally sized and periodically created at constant intervals and face tight delay and jitter requirements.
Considering these characteristics, dynamic scheduling like being used for other data services would lead to an enormous amount of signaling overhead. With dynamic scheduling, each user inside a cell is signaled the designated transmission resources explicitly once every scheduler interval via a special control channel. In case the scheduled data blocks are very small and numerous the control channel may reach its maximum capacity limit although the actual data channel has not reached the peak of its capacity. Furthermore, the dynamic scheduling of equally sized VoIP packets leads to large amounts of redundant signaling since explicit signaling is used for each packet in order to allocate the same amount of resources at a constant time interval.
One way to tackle the problem of increasing the signaling overhead is to employ a scheme called persistent scheduling. According to this scheme, a resource is allocated only once at the beginning of the recurring transmissions at specifically time intervals. During the persistent scheduled transmission, no additional signaling overhead is required after the initial resource allocation, thus reducing the control channel overhead significantly.
In case a synchronous HARQ (=Hybrid Automatic Repeat Request) retransmission scheme is used as is the case with UMTS LTE uplink, it is possible that the HARQ retransmission of dynamically scheduled data collides with a persistently scheduled resource (LTE=Long-Term Evolution). To avoid collisions, a persistent scheduled allocation may be shifted in time and/or frequency.